1. Field of the Invention
The invention relates to wavelength division multiplex fiber optic transmission systems. To be more precise, the invention relates to an extractor for extracting a selected frequency from a wavelength division multiplex signal containing N frequencies and to a frequency switch and reconfigurable frequency add and drop multiplexer.
The context of the invention is therefore that of optical switching architectures.
2. Description of the Prior Art
Identical frequencies transporting different information for different destinations arrive at optical switching nodes. It is necessary to route the information to the respective destinations at the switching node.
This is done by optical switches which add and drop optical channels.
Many add and drop multiplexers and switches of this type are already known in the art.
For example, FIG. 1 is a diagram showing a multiplexer for multiplexing N channels. The multiplexer is described in the paper “Integrated Multichannel Optical Wavelength Selective Switches Incorporating an Arrayed Waveguide Grating Multiplexer and Thermooptic Switches”, published in April 1998 in the review J. of Lightwave Technol., vol. 16, No. 4, pages 650–655.
The multiplexer shown in FIG. 1 includes an arrayed waveguide grating (AWG) 3. The arrayed waveguide grating 3 has 2N+2 input ports 1 to 2N+2 and 2N+2 output ports 1 to 2N+2. An array of N 2×2 optical switches SW1 to SWN interconnects the various input and output ports of the arrayed waveguide grating 3 via loopback lines 4.
In a first input multiplex M1 information i1, i2, . . . , iN is coded on respective frequencies f1, f2, . . . , fN and in a second input multiplex M2 information i′1, i′2, . . . , i′N is coded on respective frequencies f1, f2, . . . , fN.
Thus the same frequencies f1 to fN carry different information. Information coded on a particular frequency is extracted and thereafter other information on the same frequency is inserted.
In the FIG. 1 example, a typical requirement is to recover information i2 coded on the frequency f2 and information iN coded on the frequency fN and to code information i′2 and i′N on the same frequencies instead.
The principle of operation for this particular example is based on the fact that the frequencies of the first multiplex that enter at the input port 1 will be demultiplexed to the output ports N+3 to 2N+2 and that all the frequencies of the second multiplex that enter at the port N+2 are demultiplexed to the output ports 2 to N+1.
The signals demultiplexed in this way are then guided toward the N 2×2 optical switches SW1 to SWN. Signals with the same frequency fi from the two input multiplexes M1 and M2 are forwarded to the same 2×2 switch SWi. The output signals switched by the N 2×2 switches are then looped back to the input ports of the arrayed waveguide grating 3.
The signals looped back to the input ports N+3 to 2N+2, on the one hand, and the input ports 2 to N+1, on the other hand, are automatically remultiplexed and forwarded to the respective output ports 1 and N+2 of the arrayed waveguide grating.
For a particular frequency fi, each switch SWi forwards the signal coded on that frequency fi either to the first set of input ports 2 to N+1, to be more precise to the input port i+1, or to the second set of input ports N+3 to 2N+2, to be more precise to the input port i+N+2. Each switch SWi therefore in fact changes the output port number 1 or N+2 to which the information coded at the frequency fi will be forwarded.
Also, if only one input port is used, a simple frequency extraction function is implemented, because each frequency constituting the input multiplex can be forwarded independently to one of the two output ports 1 or N+2, depending on the switch configuration adopted.
However, the FIG. 1 router, i.e. the AWG, is in no way optimized in terms of the number of channels. To be able to process N channels with this kind of architecture, a router that is capable of routing 2N+2 channels is required. The arrayed waveguide grating is therefore overspecified as it must have 2N+2 input ports and 2N+2 output ports to process N frequencies.
Consequently, this amounts to an approximate doubling of the number of waveguides in the arrayed waveguide grating, which makes the system complex and costly.
Another drawback of the above solution is that it is based on the use of 2×2 optical switches.
If the switches used are thermooptic switches, the operating speed is limited. With switches of this type, it is not possible to select frequency coded information in less than a few nanoseconds.
Switches based on optical amplifiers can be used for faster operation, however. Nevertheless, to produce a 2×2 switch based on optical amplifiers, it is necessary to use four active components, as against only two for a thermo-optical switch. Accordingly, in the FIG. 1 example, if a fast solution is implemented based on optical amplifiers, it is necessary to provide 4N active components, which represents a penalty in terms of power consumption.
Thus regardless of the technology employed, using 2×2 optical switches is unsatisfactory.
Consequently, the object of the present invention is to provide a compact and fast system that is capable of selectively extracting one or more frequencies from a wavelength division multiplex input signal, i.e. that is capable of relaying all the input frequencies with the exception of at least one selected frequency to the same output port and the selected frequency or frequencies to another output port.
The invention also proposes to expand the architecture of the extractor to encompass more complex functions, and in particular to provide a reconfigurable frequency add and drop multiplexing function that alleviates the drawbacks of the prior art previously cited.
To this end, the invention exploits the routing properties of arrayed waveguide grating multiplexers to extend the architecture of a conventional wavelength selector to an architecture including a plurality of stages of interleaved optical switches.
The incoming wavelength division multiplex optical spectrum is therefore divided by a first demultiplexer and forwarded to a plurality of interleaved stages of optical switches for selectively feeding a plurality of input ports of a multiplexer, whose routing properties are then used to implement the complex functions referred to above, i.e. extracting one or more frequencies from the input multiplex, reconfigurable frequency add and drop multiplexing, or reconfigurable frequency switching.